Tools and methods for producing nanoantenna electronic devices

ABSTRACT

The present disclosure advances the art by providing a method and system for forming electronic devices. In particular, and by example only, methods are described for forming devices for harvesting energy in the terahertz frequency range on flexible substrates, wherein the methods provide favorable accuracy in registration of the various device elements and facilitate low-cost R2R manufacturing.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/447,807 filed Mar. 2, 2017 entitled TOOLS AND METHODS FOR PRODUCINGNANOANTENNA ELECTRONIC DEVICES which is a continuation of U.S. patentapplication Ser. No. 14/281,583 filed May 19, 2014 entitled TOOLS ANDMETHODS FOR PRODUCING NANOANTENNA ELECTRONIC DEVICES U.S., which claimspriority to U.S. Provisional Patent Application No. 61/824,746 filed May17, 2013 entitled TOOLS AND METHODS FOR PRODUCING NANOANTENNA ELECTRONICDEVICES, and is related to Ser. No. 11/337,013 filed Jan. 20, 2006entitled REPLICATION TOOLS AND RELATED FABRICATION METHODS ANDAPPARATUS, and Ser. No. 11/471,223 filed Jun. 20, 2006 entitled SYSTEMSAND METHODS FOR ROLL-TO-ROLL PATTERNING, and Ser. No. 11/711,928 filedFeb. 27, 2007 entitled FORMATION OF PATTERN REPLICATING TOOLS, and Ser.No. 11/830,718 filed Jul. 30, 2007 entitled ADDRESSABLE FLEXIBLEPATTERNS, and Ser. No. 12/270,650 filed Nov. 13, 2008 entitled METHODSAND SYSTEMS FOR FORMING FLEXIBLE MULTILAYER STRUCTURES, the entirecontents of all of which applications including the ProvisionalApplication are incorporated by reference herein.

FIELD OF INVENTION

The present invention relates generally to the field of formingelectronic devices and more particularly to the field of terahertzenergy harvesting devices.

BACKGROUND

Rectennas, rectifying antennas that convert incident radiation intoelectricity, have been investigated for a number of years, and opticalrectennas for converting solar radiation into electricity have also beenproposed^(i,ii,iii). These devices are generally comprised of two keyelements: an antenna array tuned to absorb incident radiation and anarray of rectifying elements that converts the antenna's high frequencyradiation into low frequency or DC electricity. Proper impedancematching is required to efficiently extract power from these devices,and at radio frequencies (RF), conversion efficiencies of over 85% havebeen demonstrated^(i). Optical rectennas, also called nanoantennas,nantennas, nanoantenna electronic collectors (NECs), or rectenna solarcells (RSCs), are of particular interest due to their theoretically highefficiencies and ability to harvest longer wavelength radiation thanconventional PV cells, such as IR and waste heat.

However, rectennas designed to harvest even longwave IR require antennaelements whose dimensions are commensurate with the radiation to beharvested, typically in the micron regime or below. The small wires thatcomprise the antenna elements may be even smaller, e.g., in thesubmicron size range. In addition, in order to rectify electronoscillations in the antenna, the rectifying elements must operate atvery high (terahertz) frequencies. For example, to convert 10 μm(longwave IR) radiation, rectenna diodes must operate near 30 THz, andat 300 THz for 1 μm (near IR) radiation.

In order for rectenna devices to become commercially viable, they, likeconventional PV modules, must be manufacturable on a scale that allowsthem to cover a large enough surface area to produce useful electricalpower, and at a cost that is competitive with other energy conversiondevices. However, current manufacturing technology faces a number ofbarriers to meeting these requirements.

Nanoscale electronic devices today require a state-of-the-artsemiconductor fabrication facility (‘fab’). These fabs are built aroundthe use of silicon wafer substrates, typically in the diameter range of150 mm to 450 mm (6-in to 18-in). The costs of these substrates aloneare high enough to make these devices unaffordable, while even themaximum substrate size is far from the square meters require for usefulenergy harvesting. Additionally, the very high facilities capitalexpense and relatively low throughput of the required lithographyprocesses prevent wafer-based nanoantenna devices from becomingpractical or affordable in the foreseeable future.

Over the last several decades, a relatively new form of patterning hasbeen increasingly applied to lithographic processing, variously calledmicroembossing, imprinting, or more recently, nanoimprint lithography(NIL)^(iv,v,vi), it replaces expensive ultra-short wavelength photomasklithography with ‘mechanical’ patterning capable of forming structuresbelow 10 nm. When carried out using roll-to-roll (‘R2R’) processing,this type of patterning shows great promise in enabling significantlyreduced device cycle times and costs for manufacturing large areananoantenna solar cells and related devices.

However, while nanoimprinting may be adequate for creating single levelpatterns, more complex multi-level electronic devices present a problem.One basic semiconductor fab process—pattern registration using maskalignment—cannot be used for forming the multiple levels of alignedpatterns at the resolution required for rectenna device formation onmost flexible polymeric substrates due to the characteristic dimensionalinstabilities of plastic films relative to silicon or glass substrates.This can be seen if attempting to align two films independently formedwith the exact pattern, even if on the same type of plastic substrateusing the same imprint tool—where one area might exhibit very goodalignment, the x, y positional errors at nearby locations can showsignificant non-linear variations, make alignment over large areasimpossible by conventional means.

Several approaches have been developed to get around this dimensionalstability issue, including various forms of self-aligned lithography andimprinting^(vii,viii,ix,x), methods which are particularly useful forroll-to-roll fabrication of flexible electronics. The advantage ofself-alignment for polymeric films is that the relative alignmentaccuracy of the features in the various device layers is established inthe master template from which the imprint stamp is ultimately made andis thus effectively independent of the non-uniform dimensionaldistortions characteristic of virtually all plastic films (U.S. Pat. No.6,861,365: Method and system for forming a semiconductor device;Taussig; U.S. Pat. No. 7,195,950: Forming a plurality of thin-filmdevices; Taussig; U.S. Pat. No. 8,263,433: Method of fabricating anactive device array and fabricating an organic light emitting diodearray Yeh, et al; Sharma: U.S. Pat. No. 7,470,544: Sensor array usingSAIL, etc.).

However, these processes suffer from serious limitations: in particular,self-aligned imprint lithography can only produce aligned patterns onmaterial layers (metals, dielectrics) that have been deposited prior toformation of the self-aligned imprint mask. For example, certain circuitrequirements, such as wire traces connecting from one layer to another,certain geometries (wire crossings), or material compatibility issuescannot be formed using this approach, while an additional layer ofcircuit elements cannot be added in registration to a multi-level devicestructure previously formed using the self-aligned process.

Attempts to get around these constraints are very limited and haveserious drawbacks. In self-aligned imprint lithography, forming isolatedelectrical contacts in a single metal layer requires formingperforations in certain locations of the overlying layers and usingchemical etching through these perforations to undercut and eventuallyseparate the underlying metal areas. This approach has the disadvantageof producing an unsupported void space under the dielectric where themetal was removed, which can result in shorting or fracturing of thedevice if flexed. Also, the undercutting technique is not verycontrollable and, other than being used to break contact betweensections of a conductive layer, cannot be used to produce thewell-defined features required in many electronic devices. Finally,“isolating” circuit traces by this technique is limited to open areaswhere the perforations are accessible for chemical etching.

Thus for R2R processing, the current approaches for fabricatingmulti-layer electronic devices in general have significant drawbacks.This is particularly problematic for rectenna devices, as they cannot beformed cost-effectively by any current means due to the their submicronalignment requirements and large size (many square feet) requirements.Given the potential utility of such devices in energy and heatharvesting, it is therefore of considerable commercial usefulness toprovide an alternative manufacturing method that can overcome theselimitations, in particular through the use of roll-to-roll processingfor achieving large area, high volume production of low-cost energyharvesting devices. These limitations, as well as others described inthe following sections, are overcome by the means of the currentdisclosure.

SUMMARY OF DISCLOSURE

The present disclosure advances the art by providing a method and systemfor forming electronic devices. In particular, and by example only,methods are described for forming devices for harvesting energy in theterahertz frequency range on flexible substrates, wherein the methodsprovide favorable accuracy in registration of the various deviceelements and facilitate low-cost R2R manufacturing.

For useful background in describing the present disclosure, two keyterms—subtractive and additive processing—will be first defined. Aswidely practiced in the semiconductor device fabrication industry, asubtractive process is generally one in which a photoresist mask isformed over a metal or dielectric material layer on a substrate, and theareas of the material layer revealed through the openings of the maskare removed, using any of several removal techniques (typically plasmaor chemical etching). The removal of the photomask (‘liftoff’) revealsthe material layer now having the desired (mask) pattern. A additiveprocess is one in which a photoresist mask is formed on a substrate orlayer and the desired metal or dielectric material deposited over theentire mask surface, including the areas of substrate exposed throughthe mask openings. Again, liftoff of the photomask, this time with theexcess overcoated deposited material, reveals the desired materialpattern on the substrate.

One embodiment of the disclosure includes a method and system forforming a multi-level device using a combination of both self-alignedsubtractive and additive processes. The multi-level polymeric imprintmask (‘ML’ mask) of the current disclosure contains levels used for bothtypes of processes. Said mask is formed on a substrate comprisingmultiple pre-deposited layers of thin-film metal and dielectricmaterials in a specific order to be patterned by the ML mask. Using anyof the known methods of subtractive processing, typically plasma orother gas-phase (‘dry’) etching process or chemical (wet) etching, theareas of the pre-deposited layer exposed through the openings of the MLmask are removed. To transfer the same pattern to additional underlyinglayers, if desired, the etch process is allowed to continue, modifyingthe etch formulation as necessary for the removal of each subsequentlayer. Depending on the specificity of the selected etch process and therequirements of the pattern, specific material layers can be designed toact as ‘stop’ layers to terminate the etch process.

For the purposes of the device fabrication method described herein, asubtractive pattern cycle will be defined as consisting of a plasma etchstep to modify the openings of the imprint mask by removing one maskheight level, thereby revealing an underlying material layer, followedby a second etch step to remove one or more layers of the materialexposed through the thus formed openings of the imprint mask. The cycleis repeated until all desired layers have been patterned. This, then,completes the subtractive patterning aspect of the pre-depositedmaterial layers.

In the preferred embodiment of the present disclosure, additionalpatterned material layers, such as an upper electrode contact layer, areformed in registration with the subtractively-formed material layers bya series of additive process steps now described. As part of the MLsubtractive patterning process, an additional mask level is included inthe ML mask and an additional opaque material layer is included in thepre-deposited layer stack. This layer does not function as a devicelayer but rather as an in situ self-aligned dark field photomask. Thislayer is formed from a metal layer such as Cr or other opaque material,and if the former, where the layer immediately overlying this layer isalso a metal, an electric isolation (dielectric) layer is included inthe pre-deposited stack to prevent shorting. The Cr or other opaquematerial mask formed by the self-aligned subtractive process is thenused in a self-aligned additive process to form a desired upperpatterned layer, accurately aligned to the previously formed devicelayers.

This is accomplished by coating a radiation curable liquid over thepreviously patterned structure and exposing this liquid through theopenings in said Cr (opaque) mask, causing the liquid to solidify onlyin those areas exposed to the radiation to which the liquid issensitive. The non-solidified portion of the liquid is washed away withappropriate solvent. In a second step, the original patterned device,along with the newly formed (‘hard’) polymeric structure, is thenplanarized with a protective (‘soft’) non-radiation sensitive polymer. Aplasma etch step is used to etch back any excess (first or second)polymer material, as necessary, and thereby expose the material featureto which the added electrode layer is to be in contact. The layer to beadded, in this example a metal for an upper electrode, is now blanketdeposited by any of several means known to the art (vacuum deposition,CVD, electroless, etc., alone or in combination). The excess depositedmetal is then removed by a liftoff step, using a solvent designed todissolve the soft polymer.

Further, it may be advantageous for certain applications, such as for amore transparent device, to remove the self-aligned opaque mask layer,therefore it is an aspect of the present disclosure to provide a releaselayer between the flexible substrate and the photomask layer (the firstlayer of the pre-deposited stack), and using a bonding adhesive, totransfer the complete device to a receiving substrate (e.g., glass,plastic, metal; rigid or flexible), thereby exposing the photomask layerfor removal by etching.

From the above discussion, it may be appreciated that the stack ofpre-deposited materials, including metals and dielectric materials, willbe optically opaque, thus the very desirable use of radiation curing toform the ML imprint mask can only be carried out if the imprint toolitself is transparent. Forming a durable but transparent ML imprint toolpresents a problem for large volume production, since the typicaloptions for such tools, including etched glass, polydimethylsiloxane(PDMS) or other transparent polymer, are typically either veryexpensive, very fragile, or both, particularly when formed into acylinder, as typically used for R2R manufacturing. It is thereforeanother embodiment of the method and process of the present disclosureto provide an ML imprint tool that is accurate, readily formed,inexpensive and suitable for production. In this embodiment, an originalmaster template having the multi-level geometry is formed by any of anumber of known techniques (e-beam or laser lithography, etc.). Themaster template is converted into durable Ni imprint tooling by knownprocesses, such as precision Ni electroforming as practiced by theCD.DVD manufacturing industry. Such Ni imprinting tools (also known as“shims”), when formed into a drum, can be used to produce a largequantity (i.e., rolls) of high quality, inexpensive but accurate polymerreplicas. These replicas, wrapped and bonded (for example, with anoptically clear pressure sensitive or other adhesive) to a transparentcylindrical mandrel, are then used in the ML imprinting process to formthe ML imprint mask used in the previously described process. Thesignificance of this embodiment is that the plastic ML tool is extremelyinexpensive and moderately durable: it can be made from robust UV resinon a polyester or other support film, or formed directly in a lowadhesion plastic, for example a cyclic olefin copolymer such asZeonor^(xi). Being inexpensive allows this plastic imprint tool(stamper) to be easily replaced whenever necessary, even after runs asshort as a single production shift. Another advantage of this embodimentis that the plastic ML tool, being transparent, can itself be made fromdurable and relatively inexpensive Ni tooling, thereby allowing largequantities of the ‘disposable’ plastic stamper to be made inexpensively.

In addition to the embodiment of the method and process of the presentdisclosure described above for radiation imprinting, in yet anotherembodiment of the present disclosure, the radiation process may bereplaced by a polymer coating process whereby a thin polymer layer, suchas cellulose acetate butyrate, polycarbonate, acrylate, or otherpolymeric material, is coated over the topmost of the pre-depositedmaterial layers. Instead of radiation (UV, visible, etc.) solidification(e.g., by cross-linking) of a liquid material applied at the imprintstation, said polymer can be coated “off-line”, as part of the workingsubstrate mentioned above. It is an aspect of the current method thatthis layer can be imprinted thermally, by softening with heat prior toor during contact with the imprint drum, but in a preferred embodiment,through the use of a solvent or solvent/diluent mixture suitable forsoftening the polymer layer for imprinting. This has the advantages overradiation and thermal imprinting that is does not require elevatedtemperatures, which can damage sensitive structures or substrates, anddoes not require optical transparency of either the working substrate orthe imprint drum, thereby enabling the use of low-cost, durable Niimprint tooling. It does require, however, a separate coating step.

In another embodiment of the method and process of the presentdisclosure for operation in higher temperature environments (such as maybe advantageous in harvesting long wavelength energy, including wasteheat), the electronic devices made by the present disclosure can beformed on metal foils, such as stainless steel, etc., or flexible glass.Since the pre-deposited material stack formed by vacuum deposition coverthe entire surface of the substrate, the processes used will not be indirect contact with the substrate and will not require modification.

The uniformity and edge acuity, etc., of the patterned material layersdepend upon the quality of the imprinted mask, and this in turn requiresa very precisely controlled polymer layer to insure precise etch removalof each of the mask levels: if a mask layer is too thick and notentirely removed, un-etched material will result, causing possibleshorts between lines. Conversely, if the layer is too thin, it mayprematurely etch and undesired material removal will result. It istherefore another embodiment of the method and process of the presentdisclosure to provide a ML mask formed by means of precisely controllingthe distributing of mask-forming polymer through the use of acomputer-controlled multi-nozzle ink jet heat array applicator. Such anapplicator can produce a high-resolution pattern of very uniform finespatially controlled dots of imprint polymer in a pattern that issynchronized in spatial with the pattern on the imprint drum on themoving web. Contact with the imprint stamp causes the drops to mergeinto a uniform film, which in turn provides a highly uniform residualpolymer layer. Spatial control of the fluid distribution allows theoptimal volume of resin to be delivered to each part of the ML stamppattern, thus for example no polymer is applied to areas where nomasking is required, or conversely, more fluid is deposited in areaswhere the ML stamp has features that require a larger fluid volume. Highuniformity imprint residue layer thickness results in higher plasma etchconsistency, more efficient use of resist, and faster etching (lessorganic material to remove). (W. Dennis Slafer U.S. Pat. No. 9,724,849:FLUID APPLICATION METHOD FOR IMPROVED ROLL-TO-ROLL PATTERN FORMATION).

It is another embodiment of the present invention to further improve theuniformity of the ML mask, while also reducing manufacturing processcost and complexity, through the use of semi-transparent nanoimprinttools^(xii) (US20120125880A1: TOOLS AND METHODS FOR FORMINGSEMI-TRANSPARENT PATTERNING MASKS SLAFER) to eliminate the residue layertypically formed as a by-product of the nanoimprint process. Thisreduces the number of plasma etch process steps and eliminates apotential source of defects—the under- or over-etching of the maskresidue layer (equivalent to the well-known ‘scum’ layer insemiconductor photoresist processing).

In yet another embodiment of the present disclosure, a means to furtherimprove the formation of the ML mask is by using a non-heating, energyefficient and non-polluting solid-state radiation sources for curing theML-forming liquid. Conventionally, radiation curing is accomplishedusing a short wavelength (mercury lamp, etc.) UV sources. This, however,has a number of drawbacks, particularly in that said sources producevery large amount of unusable heat and UV radiation (i.e., outside ofthe wavelength band that contributes to imprint mask curing), wastingenergy (increasing costs) and in many cases forming ozone as aby-product that, along with eye danger, poses potential threats toworkers. Replacement of these radiation sources in the presentdisclosure with low current, narrow band (<10 nm) output solid-state LEDlight sources operating in the long wavelength UV/visible spectralregion, and sensitizing the photocurable mask formulation to solidify atthe peak wavelength output of such diodes (e.g., 390 nm diodemanufacturer Infinilux, Inc., Commerce Calif.), allows virtually allradiant energy to be fully utilized, enabling the R2R process to runfaster while avoiding the common thermal substrate distortion commonwith conventional high-powered UV lamps.

Other aspects and advantages of the present disclosure will becomeapparent from the following detailed description, taken together withthe accompanying (not to scale) drawings, illustrating by way of examplethe principles embodied in the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematic drawings of subtractive patterning process usingan imprinted mask [PRIOR ART].

FIG. 2 shows schematic drawings of additive patterning process using animprinted mask [PRIOR ART].

FIG. 3 is a schematic showing a transparent drum with bonded flexiblestamp for imprinting a multi-level pattern. Also shown is a stationaryinternal LED radiation source for curing imprint fluid throughtransparent R2R drum tool.

FIG. 4 is a schematic drawing of a machine for roll-to-roll (R2R)formation of ML imprint masks.

FIG. 5 is a detailed view of a component of a R2R machine for forming MLimprint masks.

FIG. 6 is a cross-sectional sketch of a multi-level polymer imprint maskon a pre-deposited multilayer stack on a substrate.

FIG. 7 is a set of cross-sectional sketches showing the series of stepsused in the formation of a nanoantenna device.

FIG. 8 is a set of cross-sectional sketches, continuing the processesfrom FIG. 7, which shows the additive process steps used in theformation of a nanoantenna device.

FIG. 9 is a set of cross-sectional sketches, continuing the processesfrom FIG. 8, which shows the final set of steps used in the formation ofa nanoantenna device.

FIG. 10 is a 3-D representation of a structure similar to that given inFIG. 7 showing a multi-level polymer imprint mask on a multilayer coatedsubstrate, where the device structure includes additional metal layers.

FIGS. 11-13 are a series of 3-D representations of one embodiment of aprocess for making nanoantenna solar cells using the material stackshown in FIG. 10.

DETAILED DESCRIPTIONS OF EXEMPLARY EMBODIMENTS OF DISCLOSURE

Detailed Description of Figures

The following description is presented to enable one of ordinary skillin the art to make and use the invention disclosed herein, and it willbe appreciated that the exemplary embodiments and principles describedin the present invention may be equally applied to other types ofelectronic devices. This section describes in detail the methods of thecurrent invention for a R2R process capable of large-scale, low costmanufacturing for rectenna and other nanoantenna devices.

For ease of explanation, the method for forming nanoantenna devices andthe like have been broken into a series of individual operations orsteps:

-   -   Step #1: coat metal and dielectric layer stack on polymer        substrate on pre-coated substrate    -   Step #2: form multi-level self-alignment mask, by nanoimprinting        etc., on pre-coated substrate    -   Step #3: use subtractive processing with ML mask and        pre-deposited layers to carry out multiple [material layer+mask        level etch] cycles until all layers have been patterned    -   Step #4: use additive processing to form additional material        layers using self-aligned internal photomask    -   Step #5: (optional): remove internal photomask and/or transfer        device to different substrate

A detailed description of the sequence of individual process steps forseveral embodiments is now given. Given in FIGS. 1 & 2 are methods knownto the art (described above) for using an imprinted polymer mask tocarry out a subtractive process for removing material and an additiveprocess for depositing material, respectively, to form patternedmaterial patterns.

FIG. 3 shows a transparent patterning drum (300) comprising atransparent sleeve (glass or plastic) bonded to a flexible ML polymericfilm layer (301) used in a roll-to-roll imprint machine (c.f. FIG. 4,related applications incorporated herein) developed for replicating suchstamp patterns. Also shown is an exposure device 303 that is mountedinside transparent drum 300 and includes a stationary LED radiationsource 304 mounted on heat sink 305 and using a roller bearing mechanism306 for allowing the transparent drum stamp to rotate circumferentiallyaround the stationary light source, allowing the radiation from the LEDsource to cure and harden the imprint polymer that is laminated betweenthe drum surface and the opaque carrier film.

FIG. 4 shows a schematic of a R2R machine that incorporates thetransparent imprinting drum 303 with internal exposure (c.f. FIG. 3) forforming the ML imprint mask on the pre-deposited flexible substrate.Machine is enclosed in clean module 400, with unwind station 401 andrewind station 402, representational fluid applicator 403, cleaningstation 404, and optical pattern measurement station 405. One embodimentof a method by which the electronic devices of this disclosure may beproduced by R2R means is discussed in detail below.

R2R Processing: Multi-Layer Pre-Deposition

Flexible substrates used in the R2R machines described in the currentdisclosure include PET (polyethylene terephthalate, or ‘Mylar’), asubstrate used commonly in R2R manufacturing because of its physicalstrength, high optical quality, chemical resistance and low cost. It isavailable in a wide range of thicknesses (4 μm to 750 μm), surfacefinishes, and surface treatments (e.g., adhesion promoting sublayers,etc.). While the typical maximum working temperature of PET isapproximately 150° C.^(xiii), a variant named PEN (polyethylenenaphthalate, 200° C.^(xiv)), as well as other commercially availablefilms (polyimide films^(xv) such as Kapton, metal foils, flexible glass)are available where process or usage conditions require a higher workingtemperatures and may also be used in the present process.

R2R ML Mask Formation

The embodiments of the present disclosure, as previously described, maybe beneficially carried out using one or more R2R processes discussed inthis section.

In a first R2R machine pass, all of the layers to be patterned by the MLimprint mask are coated in a specific order, typically by sputter orother vacuum coating, although any of the means known to means arepossible as well. This coated film, along with the layers subsequentlyadded or processed, will be referred to as the “working substrate”. InFIG. 4, the spool of working substrate 401 is fed into the imprint zone303, after passing through cleaning zone 404 to remove particulate orother contamination. The working substrate enters the imprint zone,where the liquid monomer resist for the ML mask is applied, either tothe drum or incoming substrate, as a controlled thin layer by ink jet orother precise coating means. Because the pre-deposited layers cause thesubstrate to be optically opaque, one embodiment of the ML maskformation process—radiation curing—must be done through a transparentimprinting stamp (tool), shown in FIG. 3 as sleeve 300 with imprintlayer 301. In this embodiment, a solid state light source 303 consistsof a stationary LED array 304 attached to heat sink 305 around whichrotates the transparent imprint sleeve by means of bearing assembly 306(FIG. 3).

The imprint zone is shown in detail in FIG. 5, wherein working substrate501 is brought in contact with the patterned surface 504 of imprintingsleeve 303. The fluid used to form the ML mask is applied by source 403(ink jet nozzle array or other), while light shield 505 preventspremature curing of liquid. The use of digitally controlled imprintfluid dispenser (ink jet nozzle array) can optimize the pattern-wisedistribution of imprint fluid to improve uniformity and fluid usage.Elastomeric input nip roller 502 presses the working substrate againstthe pattern surface (504) while under exposure to radiation from theinternal LED source. Now patterned substrate 503 exits the imprint zoneand travels through an inspection station 405 (FIG. 4) where an opticalmeasurement system (camera or laser based) evaluates the quality of theimprinted layer. As previously seen (FIG. 4), it travels to the take-upspool 402, with the rewound working substrate now consisting of animprinted multi-level resist mask adhered to a multilayer thin filmstack on a flexible substrate.

R2R Plasma and Chemical Etching

The next R2R machine pass (not illustrated) removes the polymer residuefrom the imprint process (where this step may be eliminated by the useof semi-transparent imprint mask tools, incorporated herein byreference) or selectively removes one or more of the pre-depositedlayers in reverse order of their deposition. Removal is carried by thetechniques of plasma or chemical etching known to the art, where theplasma process is used for removing polymeric material, such as the maskresidue and/or polymer mask levels. The etch process my include a seriesof steps with individual etchants optimized for one or more of thepre-deposited layers, thus several etch passes may be required asnecessary, although it is desirable to utilize etchants or etchantblends that will process multiple layer per pass. These steps arecarried out by chemical, plasma, or a combination of etching means. Inthe present embodiment, a R2R machine for chemical etching includes aseries of chemical immersion and rinse baths, each with the appropriatechemistry to remove a target material, and various types of end-pointdetection known to the art are used to control the etch process toeliminate under- or over-etching of pattern elements.

After all layers have been patterned through the first ML mask level bythe above means, the mask itself is etched in height to reveal thesecond mask level, which reveals another set of mask openings thoughwhich another set of etching operations is carried out. This cycle isrepeated until all of the pre-deposited layers have been patterned. Thepreferred embodiment of this disclosure includes the patterning of anopaque layer, such as Cr, that has been included as one layer of thepre-deposited stack. Because this layer is formed by the ML mask and istherefore properly aligned with the other patterned layers of thedevice, it will enable the critical formation of additional, preciselyaligned materials layers that could not be formed by the conventionalself-aligned ML imprint patterning process alone.

Exposure Through Internal Photomask

After the pre-deposited layers of the working substrate have beencompletely patterned, the R2R process shown in FIG. 4 is again used,this time to planarize the patterned film using a curable liquid used toform a structural support for the added pattern layers. The film istransported into the imprint zone 303, but in this case the transparentsleeve with the ML film overlay has been replaced by a smooth,patternless transparent drum sleeve (i.e., 300 without 301 in FIG. 3).The film is brought into contact with this clear drum so that theuncoated side of the working substrate is against the sleeve, allowingthe radiation from 304 to pass through the internal photomask beforeexposing the applied fluid. A temporary plastic film or the glass sleeveitself can be used to provide a smooth or textured surface for thisprocess. After exposure, the planarized film exits the exposure stationand (after removing the temporary strip sheet, if used), the unexposedfluid is rinsed off with appropriate solvent and dried.

Soft Polymer Application In a next step, a coating process is used toagain planarize the patterned film surface, this time with a ‘soft’(non-radiation curable) polymer that will temporarily protect the devicestructures during a subsequent material deposition step (not shown).Application of this soft polymer layer may be by solvent coating orthermal laminated or other appropriate means. At this stage, the workingsubstrate with the soft polymer top layer is rewound onto a take-upspool.

Additive Layer Deposition In the next step, a R2R vacuum process is usedto remove excess soft (and hard) polymer, as necessary, to both exposeand clean the topmost material layer—formed by the previous subtractiveself-aligned patterning process—to which the deposited layer willcontact (this aspect of the disclosure is described in more detail inFIGS. 10-11). In a sequential vacuum operation, preferably withoutbreaking vacuum, the top material is deposited after the etch-cleaningstep. This sequence will assure good electrical contact between thepreviously patterned material layer and the layer being deposited.

Liftoff & Final Steps The next R2R step, carried out under atmosphericconditions in a machine such as used for the R2R chemical etchingpreviously described, the excess soft polymer along with excess materialfrom the previous vacuum step is removed by exposing the workingsubstrate to a solvent that is appropriate to dissolve the soft polymer,thereby removing said polymer and excess vacuum deposited material,followed by rinse and drying steps.

At this point it may also be desirable to remove the internal opaquephotomask layer in order to provide a more transparent material forcertain applications or to transfer the device from the workingsubstrate to another substrate, such as one suitable for a highertemperature working environment (e.g., metal foil, flexible glass,polyimide, etc.). This is done by incorporating a release layer into thepre-deposited stack, between the substrate and the photomask layer. Sucha layer, as well known to the art, can be activated by heating, chemicalexposure, or mechanical separation. In a R2R process (not shown), theworking substrate fed from a supply spool to a laminating station wherethe patterned side is adhesively bonded to a suitable carrier film,after which the release layer is activated and the original substratedelaminated. The now-exposed internal mask layer is chemically removed(etched), as well as the now-exposed electrical insulation layer, ifdesired. The adhesive used in this lamination step can also serve asencapsulants to prevent chemical, moisture, oxygen attack, wherenecessary. Similarly, the now exposed surface of the device may also becoated with an encapsulants material for similar reasons.

The individual steps of the above-described R2R process will now bedescribed in a detailed fashion, using discrete coupons as explanatoryexamples, and it should be noted that the devices of this disclosure canbe made by either R2R or batch processes.

This completes the description of the series of R2R processes that, asone embodiment of the current disclosure, provides a means of large areaproduction of electronic devices with precisely aligned submicronfeatures, such as terahertz energy harvesting devices and the like.

In the following figure descriptions, the individual elements of thefabrication methods of the current disclosure will be described in astep-by-step basis.

In FIG. 6 is shown a cross-sectional sketch showing the initialpatterning levels and mask for producing a rectenna device based on anarray of metal nanoantennas with metal-insulator-insulator-metal (MIIM)rectifying elements, this being but a single example of possibleelectronic devices and nanoantenna-based devices that can be formed bythe means of this process. The metal-insulator1-insulator2 stack(603-604-605, respectively) together form part of a MIIM diode tunnelingjunction, a device known to the art as being capable of rectifying thehigh frequency radiation absorbed by the nanoantenna_(xvi). In thisexample, a substrate 610 has been pre-coated with a sequence of layers,including a first chrome layer 601, and electrical insulation (SiO2)layer 602, a metal electrode layer 603, a first thin diode insulator 604and a second thin diode insulator 605. The metal electrode could be anyappropriate diode metal, such as Ni or Nb, and the insulators could beoxides of these or other metals or other appropriate insulatormaterials. Over upper insulator 605 is formed the 3-level imprint maskcomprising a polymer residue layer 606, a first mask level 607, a secondmask level 608, and a third mask level 609, where the height of eachacts to separate the levels and allow removal of each level in turn byplasma etching. It should be noted that different metals and multiplelayers of metals can be used for the antenna metal and for the diodeelectrode metal, where here for the sake of simplicity is represented byonly one metal layer 603 for the lower antenna metal and lower diodeelectrode. In addition, other metals and/or dielectric layers may beadded, such as might be required to act as transition or barrier orisolation layers, including for the insulators nitrides, carbides,oxides, etc. At this point in the process, it should be noted that thetop electrode/antenna metal is not yet part of the stack.

FIG. 7 illustrates in cross-section the steps used to convert thepre-deposited material layers of FIG. 6 into the patterned metals andinsulators that make up the lower part of the device stack, although bychanging materials, thicknesses and stack ordering, the concepts of thisdisclosure can be applied to other types of electronic devices. Theprocess begins with the removal of the polymer residue (606 in step 1[upper left]) to expose the underlying upper insulator layer 605 in step2, then using plasma etching or wet (chemical) etching as known to theart, or a combination thereof, to remove all of the deposited stack oflayers exposed through the mask openings (604, 603, 602, 601 of FIG. 6),where the etch process and chemicals are adjusted, as necessary, insequence to carry out the desired removal of all material layers. Thisetch sequence stops when the substrate 610 is reached (step 3). In step4, plasma etching (usually by an oxygen-argon or other gas plasmaprocess known for removing organic material) removes the lowest masklevel (607), resulting in mask 710 remaining. The material removed fromthe mask, 700, is shown in lighter gray in this sketch. The removal ofeach step of the multi-level mask reveals new areas of the pre-depositedstack, which are in turn etched by one or more selective etch processesthat do not affect the polymer mask and are designed to stop atelectrical insulation layer 602 (step 5), thereby defining a pattern inlower metal 603. This completes the patterning cycle for mask level 1,and for the purposes of this discussion, a sequence of mask etchingfollowed by material etching will be referred to as an etch cycle. Step6 of FIG. 7 shows the result of the plasma removal of mask level 2,forming new mask profile 730 by the removal of material shown grayedout, 720.

Continuing the process in FIG. 8, the result of the second materiallayer etch process is shown (step 1, top left), which patterns the twoinsulator layers, shown as 604, 605 on lower patterned metal layer 603.The last remaining parts of the mask, 709, is removed by plasma etch(step 2), completing the subtractive steps and setting the stage for theadditive processing.

In step 3 of FIG. 8, the top surface of the device is planarized using aradiation-curable liquid (800, step 3) with lamination to a temporarycover sheet film (not shown). The laminated structure is illuminatedthrough the bottom substrate such that radiation passes through theopenings in the chrome mask layer 610 (FIG. 7), causing the polymer toharden only in the areas of the mask openings (810, step 4). The stripsheet is removed and the remaining (uncured) polymer is removed,typically by solvent rinse, defining the plateau 810 to be used forsubsequent metal deposition and exposing areas 814 (step 5). Anyhardened material 810 above the top level of the upper insulator (605)can be plasma etched to reduce this height (step 6, 820). A secondplanarization layer (step 7, 840) is applied over the device surface,this time using a non-crosslinkable (‘soft’) polymer, such as acrylic orpolycarbonate or CAB (cellulose acetate butyrate, etc.), PVA (polyvinylalcohol), etc. This layer is used to temporarily protect the other partsof the device from the upcoming metallization step. Another etchcleaning step (not shown) is used to expose and clean the upperinsulator 605, after which one or more layers of upper electrode/antennametal(s), shown as single layer 850 in step 8) are deposited by vacuumdeposition, plating, or combination deposited over the cleaned surface.In order to define the pattern of the upper metal in layer 850, aliftoff process in which solvent is used to remove the excess softpolymer and excess overcoated deposited metal, also revealing the lowerelectrode/antenna metal layer 603. At this point the MIIM structure hasbeen formed.

Additional optional steps can be used to either apply a protectiveovercoat to the MIIM structure, to transfer the device to anothersubstrate, or to remove the opaque internal mask to make the structuremore transparent. To accomplish these objectives, in FIG. 9 is shown anadditional layer 900 (step 2) in the original layer structure. Layer 900is a release layer, as described above. In step 3, the upper surface ofthe MIIM device is laminated to a transparent substrate 920 using anadhesive 910, preferably a radiation cured adhesive for easy and rapidprocessing. After delamination (step 4), substrate 610 (FIG. 6) isseparated from the device, which is now bonded to substrate 920 (step3). At this point (step 4) metal additive mask layer 601 and insulator602 can be removed by plasma or wet etching, thereby exposing the bottommetal/antenna layer 603 (FIG. 6). This layer can then be protected byaddition of coating 930 (polymer or inorganic), shown in step 5. Theresultant device comprises one of many possible embodiment of ananoantenna/MIIM diode array for rectification of incident theradiation.

FIG. 10 shows a 3-D perspective view of a structure similar to thatshown in FIG. 6, except that in this embodiment, two additional metallayers are incorporated, thus the pre-deposited stack is as follows(starting from the top and moving down):

-   -   Metal2 (1007)    -   Insulator2 (1006)    -   Insulator1 (1005)    -   Metal1 (1004)    -   Antenna Metal (‘Ma’, 1003)    -   Insulator (1002)    -   Photomask metal (1001)    -   Substrate (1010)

The ML imprint mask and the residue layer resulting from certain formsof imprint processing are shown as 1015 and 1016, respectively.Regardless of the additional layers in the stack relative to the devicein FIG. 6, the sequence of processing steps is essentially equivalent tothat described previously. The use of 3-D perspective in the descriptionof this embodiment is also meant to aid the reader in betterunderstanding the process.

Beginning with the structure shown in FIG. 10, the sequence of stepsused to pattern the pre-deposited layers and prepare for additiveprocessing is given in FIG. 11. Note that as in previous illustrations,the sequence starts at the upper left of the illustrations and each stepsequentially follows the arrows. ML mask 1015 is first formed overpre-deposited stack 1110 and subsequently plasma etched (1111), using areactive ion etch process with argon and oxygen gas, resulting in thetop metal (M2) being revealed (1102) in step 2. The third step shows theculmination of a sequence of etch steps 1103, either chemical or plasma,or a combination of both, that patterns layers M2, I2, I1, M1 and Ma,but stops at the insulation layer 1002 (FIG. 10). Step 4 shows thedevice after removal of the next mask level, which reveals M2 layer1104. A next series of etch steps 1115 remove all remaining exposedportions of M2, I2, I1, M1, and Ma, the result of which is illustratedin step 5, which shows the metal/insulator stack 1122 and exposedelectric insulator layer 1120. Mask etch process 1124 is used to removethe next mask level, resulting in upper metal M2 (step 6) beingrevealed.

A selective etch sequence 1128 is used to remove portions of the exposedmetal/insulator stack not protected by mask level 1126 down to the M1layer, where the results are shown in step 7. Also shown in that step isthe last element of the multi-level mask, which has been removed in step8, revealing the upper metal contact layer M2.

In step 9 (bottom left), the surface shown in step 8 is planarized by aradiation-curable liquid 1138 laminated between said device and a planarsurface (plastic film or glass plate, not shown for clarity). Radiationexposure 1140 of the liquid through the patterned metal layer results inthe solidification of the irradiated liquid, 1142, followed by solventrinse of un-crosslinked liquid, reveals a structure that will become asupport or plateau for deposition of an additional metal layer byadditive processing.

FIG. 12 shows the next part of the additive process in which anotherplanarizing liquid polymer 1150, this not radiation curable, is coatedover the device and solidified, and etch process 1152 is then used toremoved excess polymer and reveal the upper surface of M2 layer, shownin step 2 as 1154, and the exposed and cleaned plateau as 1156. Thisaspect of the current disclosure is significant because the etchprocess, carried out in vacuum, also serves to clean and prepare theupper metal contact 1154 for subsequent coating, also in vacuum and donepreferably without breaking vacuum between these steps.

In step 3, a metal layer 1158 is blanket then deposited over the topsurface by any of a metal deposition means (sputtering preferred, butalso e-beam, thermal vacuum deposition, chemical vapor deposition (CVD),electroplating, electroless deposition, atomic layer deposition[ALD]^(xvii,xviii)). A liftoff process is carried out in step 4, whereina solvent is used to dissolve the soft polymer layer 1159, removing thismaterial along with excess metal from the deposition process, therebyproducing patterned metal layer 1160.

The finished device structure is given in step 5, showing the lowerelectrical insulation layer 1120 that isolates the underlying metalphotomask layer, the lower metal antenna/contact metal 1164, themetal/insulator MIIM stack 1166, and the top metal antenna/contact layer1162.

In certain situations it has been mentioned that it may be desirable tohave a more transparent device, or to transfer the device to anothersubstrate, such as metal foil for use in a higher temperatureenvironment. These options are illustrates in FIG. 13, wherein theoriginal device stack now includes an additional layer 1170 that acts asa release layer to enable separation of the full device stack from thesubstrate. Step 2 in FIG. 13 illustrates the lamination of a carrierfilm 1172 (a plastic film, metal foil, or flexible glass substrate) withan appropriate adhesive 1174 (UV cure, thermal, reactive or any suchadhesive) to the patterned device, followed by the separation atlocation 1176 of the original substrate, with the result (step 3)showing the device now inverted with respect to substrate 1172, andopaque photomask layer 1178 (layer 1001 in FIG. 10) thereby beingrevealed. Step 4 shows the full structure after chemical removal of 1178and exposed electrical insulator layer 1180 (1002 in FIG. 10), which canbe similarly removed or left in place. The exposed surface can beelectrically connected to other devices and/or further encapsulated forprotection.

The structure formed in this process incorporates lower and upper metalnanoantenna and contact layers and a (generic) MIIM rectifying diodearray in contact with the nanoantenna array, all of which has beenformed by self-aligned techniques that do not require mask alignment orother processes problematic for flexible substrates and by processesthat can readily be carried out using R2R machines.

It should be clear from the various embodiments described above thatmany types of electronic devices can be formed by the methods, orvariations thereof, of the present disclosure.

REFERENCES

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What is claimed is:
 1. A rectifying antenna system for convertingelectromagnetic radiation into electricity, the system comprising: a. afilm substrate having a first electrically conductive layer that isplanar and adjacent to the film substrate, wherein the firstelectrically conductive layer is configured as a first antenna structurewith associated contact leads; b. a first insulator layer that is planarand in contact with at least one portion of the first electricallyconductive layer, wherein an interface between the first electricallyconductive layer and the first insulator layer forms a first diodejunction having a first pattern; c. a second insulator layer overlyingthe first insulator layer, wherein an interface between the first andsecond insulator layers forms a second diode junction having a secondpattern, wherein the second pattern is similar to the first pattern; d.a second electrically conductive layer overlying the second insulatorlayer, wherein an interface between the second electrically conductivelayer and the second insulator layer forms a third diode junction havinga third pattern; e. a standoff structure that is insulating andcomplementary to the antenna structure component, wherein the standoffstructure is essentially planar and in direct contact with at least onepart of the second electrically conductive layer and has approximatelythe same height relative to the film substrate; and f. a thirdelectrically conductive layer in contact with an upper surface, distalfrom the film substrate, of the standoff structure and adjacent to thethird diode junction, thereby forming a second antenna structure withassociated contact leads, wherein the second antenna structure iscomplementary to the first antenna structure; g. wherein all of thelayers of (a) through (f) are configured as a layered stack ofessentially planar layers.
 2. The system of claim 1, wherein theinsulator layers are sufficiently thin and capable of allowing electrontunneling.
 3. The system of claim 1, wherein the insulator layers andadjacent electrically conductive layers form aconductor-insulator-conductor tunneling diode.
 4. The system of claim 1,wherein the electrically conductive layer nearest the substrate includestransparent and opaque areas and for functioning as a photomask.
 5. Thesystem of claim 1, wherein a transparent area has the approximate shapeof the standoff structure.